1. Field of the Invention
The present invention relates to antennas for wireless communications.
2. Background Information
Portable devices having wireless communications capabilities are currently available in several different forms, including mobile telephones and personal digital assistants (PDAs). A portable device such as a wireless modem may also be used to provide such capabilities to a laptop or other computer (e.g. via a datalink to the computer such as wired RS-232, infrared, or Bluetooth). The field of applications for such devices is expanding rapidly, including such features and functionalities as high data rates, access to Internet and e-mail services, simultaneous transmission of voice and data, and video capabilities.
One type of antenna commonly used in portable devices for wireless communications is the monopole whip. A monopole whip antenna is essentially a wire that extends along or away from the device and is fed by a printed circuit board (PCB) of the device. One problem of this unbalanced design is that RF currents induced on the PCB may cause receiver desensitization, thereby limiting the useful range of the device.
In a monopole whip design as described above, and other unbalanced designs used in similar applications, the PCB may function as a part of the antenna. As a result, the PCB may also radiate a portion of a signal being transmitted, causing operating characteristics of the antenna such as gain, radiation pattern, and driving point impedance to become dependent on qualities of the PCB such as size, shape, and proximity to other structures (such as a display, a cable, a battery pack, etc.). Therefore, it may become necessary to redesign the antenna to achieve a similar performance with different applications and/or different types of devices.
Radiation by a PCB due to RF coupling with an unbalanced antenna may also cause efficiency losses. In a mobile phone application, for example, radiation of a PCB that is placed next to the user""s head may be wasted due to absorption of the radiating fields by the user""s head and hand. In addition to reducing the efficiency of the device, this effect may also increase the specific absorption rate (SAR) beyond regulatory limits.
Inefficiencies due in part to PCB radiation may also occur in applications other than mobile telephones. For example, a wireless modem may be designed to attach to the back of the display of a laptop computer (e.g. by clip or VELCRO(copyright)). Such a modem may include dual unbalanced whip antennas (e.g. for diversity reception) mounted to extend above the display. Due to the unbalanced feeding arrangement, however, the modem PCB ground plane may become part of the radiating antenna. Hence a portion of the radiated signal may be blocked by the laptop display panel, which may cause distorted radiation patterns, reduced antenna gain, and unwanted coupling to and from electronics in the laptop display.
Use of one or more unbalanced antennas in PCMCIA card devices (such as wireless modems) may cause similar problems. In this case, a portion of the radiator is the PCMCIA card PCB ground plane, at least a part of which is enclosed within the laptop""s metal case when the card is inserted into the PCMCIA slot of the laptop. The radiating structure is then an L-shaped dipole, half of which is buried in the laptop. The potential for pattern distortions and possible EMI problems (e.g. due to unwanted coupling and blockage by the laptop structure) is high. Additionally, the performance of this type of antenna system may vary for different laptop designs, due to differences in local grounding conditions and the proximities of metal surfaces to the PCMCIA card.
A coaxial sleeve dipole is a balanced antenna that tends to de-couple the antenna system from the circuit board or device to which it is connected. Such an antenna is constructed of coaxial cable, where the center conductor extends beyond the outer conductor, and the outer conductor is rolled back to form a jacket. One advantage of this design is that if the jacket has the right length, then current which otherwise might distort the radiation pattern may be impeded from flowing along the outer surface of the feed cable. Unfortunately, coaxial sleeve dipoles are too bulky and heavy to be practical for use in small portable devices and are not compatible with the small, slim profiles of present portable wireless devices. Additionally, coaxial sleeve dipoles are relatively expensive.
The demand for wireless connectivity from portable devices is rapidly expanding. As a result, the demand for high performance, low cost, and cosmetically appealing antenna systems for such devices is also increasing. Therefore, an antenna that reduces RF coupling problems and that may be fabricated to have features such as a low profile, light weight, and low cost is desired.
An antenna according to one embodiment of the invention includes a substrate, a first conductor, and a second conductor. The substrate has two opposing surfaces, and the two conductors are disposed on opposing surfaces of the substrate. The first conductor has a feed line portion and a poise portion, while the second conductor has a ground plane portion and a counterpoise portion. In one example, the counterpoise portion includes two stubs on opposite sides of the ground plane portion, which stubs may be substantially parallel to the ground plane portion and/or may have lengths of one quarter-wavelength at a frequency within a predetermined range.
The feed line portion may be configured and arranged to operate as a microstrip transmission line, while a portion of the second conductor including the counterpoise portion may be configured and arranged to operate as a coplanar waveguide. In such case, the coplanar waveguide may be configured and arranged to impede a flow of a common mode current over a predetermined frequency range. For example, the coplanar waveguide may be configured and arranged to have a high input impedance over the predetermined range. Other embodiments of antennas, and of assemblies including features of two or more such antennas, are also described herein.